Thousands of patients suffer cardiogenic shock following heart attacks or open heart surgery. These patients may benefit from mechanical circulatory support with a minimum output of 3 liters per minute. Many patients need temporary cardiac support during emergency transportation in an ambulance.
The need for a minimally invasive mechanical heart assist device has long been recognized. An ideal device would (1) have a cross-section of 12 French or less so that it could be adapted for insertion via a peripheral artery, such as the femoral artery (2) function intra-arterially and be inserted by a cardiologist without support from a surgeon and, (3) be capable of providing at least 3 liters per minute of flow at systemic pressures without a contribution from the native left ventricle. The intra-aortic balloon pump (IABP) has been used for years and is the industry standard because it is easy to insert and does not require surgery. It is readily inserted into the femoral artery by the cardiologist, but it has limited pumping capacity and can only be used for a patient who has some residual cardiac function. The flow of the IABP is limited to approximately 1.5 liters per minute to 2.0 liters per minute and is dependent upon synchronization with a left ventricle which must have some residual function.
There have been efforts to provide a temporary, minimally invasive pump for patients which require more cardiac output than can be provided by an IABP. The Hemopump is an axial flow blood pump which meets the criteria for blood flow (approximately 5 liters per minute) but it is too large (14 to 22 French) for easy insertion by a cardiologist. Although smaller versions of the Hemopump could be built, physics limits the flow because as the pump becomes smaller, the inlet area decreases. Losses in the pump increase in a rapid, non-linear manner as the inlet area decreases. To compensate for these rapidly increasing losses, the rotor speed must be increased exponentially. Although adequate flow may be achieved, hemolysis increases to unacceptable levels.
Thus the engineer faces theoretical and technical difficulties to make a traditional propeller pump or centrifugal pump with the diameter less than 4.0 mm and a flow of at least 3 liters per minute. One way to circumvent the physical limitations imposed by a decreasing inlet area is to make the pump expandable. In this way, inlet losses and shaft speed can be minimized since large areas can be achieved after the pump is inserted. Cable driven axial flow blood pumps have been described which use a hinged propeller that deploys after insertion into the arterial system. However, hemolysis has limited the adaptation of this concept as a clinical device. Other concepts for pumps that expand or deploy after insertion have been proposed. However, these miniature expandable pumps are challenging to manufacture and reliable mechanisms may be difficult to achieve.
The Hemopump and expandable pumps have constrained pump design by dictating that the inlet area must be located in a plane perpendicular to the axis of rotation. Consequently, an increase in the inlet area will also increase the diameter of the pump.
It is, therefore, an object of the present invention to achieve the benefits of a large inlet area without the necessity for enlarging the inlet diameter or making the blood pump expandable.
It is another object of the present invention to provide a blood pump which provides low inlet and outlet losses while maintaining a small diameter and without the need for an expandable mechanism.
A still further object of the present invention is to provide a blood pump that obviates the problems discussed above that are concomitant with prior art blood pumps.
A further object of the invention is to provide a blood pump having a 3 liter per minute or greater flow and having a diameter that is small enough to permit percutaneous insertion of the pump into a patient's blood vessel.
Another object of the invention is to provide a blood pump that is relatively simple in construction and relatively easy to manufacture.
Other objects and advantages of the present invention will become apparent as the description proceeds.